1. Technical Field of the Invention
The present invention relates to the field of switching power supplies, and in particular, to a switching voltage regulator module.
2. Description of the Related Art
Advances in integrated circuit (IC) technology often relate to the ever-decreasing operating voltages required to operate such circuits. A lower operating voltage may translate into lower costs due to decreases in circuit size and power consumption. Present demands for faster and more efficient data processing have prompted a significant development effort in the area of low-voltage integrated circuits. Currently, low-voltage integrated circuits operating in the three-volt range (e.g., 3.3 V ICs) are highly desirable. The three-volt ICs are gradually replacing the standard five-volt ICs due to their higher speed and higher integration densities.
Moreover, the three-volt ICs consume less power than the traditional five-volt ICs. Thus, in battery operated devices, such as portable telephones and lap-top computers, low-voltage integrated circuits allow the devices to operate proportionally longer than devices requiring higher voltage for operation.
However, the 3.3 V ICs represent only a transition to ICs with even lower operating voltages that will not only further improve speed and reduce power consumption, but will also allow direct, single-cell battery consumption. It is expected that the next generation of data processing ICs will be operable at voltages in the 1-2 V range. At the same time, since more devices are integrated on a single processor chip and the processors operate at higher frequencies, microprocessors require aggressive power management. Compared with current processors which require a current draw of around 13 amps, future generation processors will require a current draw in the range of 50-100 amps. The load range may reach 1:100.
Further, as the speed of the ICs increase, they are becoming more dynamic loads to their power supplies. Next generation microprocessors are expected to exhibit current slew rates of 50A/microsecond. Moreover, the output voltage regulation becomes much tighter (e.g., from 5% to 2%). Voltage regulator modules (VRMs) which feed the microprocessors have to have high efficiency, fast transient response and high power density. These requirements pose serious design challenges.
FIG. 1 is a schematic block diagram of a prior art synchronized buck converter 100. The circuit 100 is typically used as a VRM to meet the requirements of high efficiency, fast transient response and high power density. In operation, switches S1 and S2 turn on and off in complementary fashion. The voltage gain of the buck converter circuit 100 can be described by:
D=Vo/Vinxe2x80x83xe2x80x83(1)
where D is the duty ratio of switch S1.
As is well known in the art, the buck converter has a high efficiency and good transient response at around a duty cycle of 0.5. For a 5V input voltage and a 2V output, the duty cycle is 0.4, which is an acceptable duty cycle ratio for achieving high efficiency.
Since future VRMs will be required to provide more power to the microprocessors, the power switch must be able to deal with higher currents, which decreases efficiency. However, in accordance with the power equation, the increased power required by future microprocessors may be achieved by raising the input voltage instead, which allows the input current to be decreased, thereby reducing conduction losses. In addition, this also reduces the size of the capacitance. As such, it is preferable that VRMs have a 12V or higher input voltage. For example, the input voltage can be as high as 19V for notebook computers. According to equation (1), the duty cycle for a conventional synchronized buck converter is as small as 0.1 with a 12V input and a 12V output. A drawback of a duty cycle on the order of 0.1is that the circuit exhibits poor performance in terms of efficiency, voltage regulation and transient response.
FIG. 2 is a circuit diagram of a tapped converter circuit 200 according to the prior art. The converter circuit 200 includes a first power switch S1 connected across an unregulated DC input source, Vin. One side of power switch S1 is connected to a first winding N1 of a tightly coupled winding pair (N1, N2). The coupled winding pair.(N1, N2) is connected at junction 12 to filter capacitor Co and load RL. Filter capacitor Co and load RL are connected in parallel. Converter circuit 200 further includes a second power switch S2 connected in series with a second winding N2 of the winding pair (N1, N2). The serially connected power switch S2 and second winding N2 are connected in parallel with the filter capacitor Co and load RL. 
The operation of the converter circuit 200 will be described with reference to FIGS. 3a-3g which illustrates the corresponding switching waveforms associated with the converter circuit 200.
At a time prior to time t1, switch S1 is OFF. From a time t1 to a time t2, switch S1 is turned ON (see FIG. 31a) and switch S2 is turned off (see FIG. 3b). The voltage difference between the input voltage Vin and the output voltage Vo, i.e., (Vin,xe2x88x92Vo) is applied to winding N1 of the coupled inductor windings N1 and N2. The input current is1, which is the winding current in N1, increases linearly as shown in FIG. 3c. Therefore, during the time t1 to t2, the input voltage delivers power to the output through the conduction of switch S1 and winding N1. During this time, energy is stored in winding N1.
At a time equal to t2, switch S1 is turned OFF and switch S2 is turned ON. The energy stored in winding N1 from time t1 to t2 is transferred to winding N2. The winding current iN2 flows through switch S2 to release its energy to the output. The process operates as a flyback converter. Based on the voltage-second balance in winding N1, the voltage gain of the converter circuit 200 can be written as:
Vo/Vin=1/[1+(N1/N2)*(1/Dxe2x88x921)]xe2x80x83xe2x80x83(2)
where D is the duty ratio of switch S1. From equation (2) it can be seen that a duty cycle on the order of 0.5can be realized to achieve a high circuit efficiency by properly choosing the turns ratio of the coupled inductors. As one example, for an input voltage, Vin=12V, an output voltage, Vo=1.5V, and a desired duty cycle, D=0.5, the ratio N1/N2 =7.
One disadvantage of circuit 200 is that a high voltage spike occurs across switch S1 when S1 turns off (e.g., at time t2, see FIG. 3f) because the leakage energy of winding N1 cannot be transferred to winding N2 due to an imperfect coupling between windings N1 and N2. The leakage energy stored in leakage inductor Lk (not shown) which cannot be transferred to winding N2 charges the output parasitic capacitance (not shown) of switch S1 through conducting switch S2 which causes a high voltage stress across S1. As a result, a high voltage rated MOSFET switch must be used in the circuit 200 which significantly increases the power loss and reduces the efficiency.
It would be desirable to provide a circuit configuration which avoids the necessity of using a high voltage rated MOSFET switch and which recycles the leakage energy of the coupled leakage inductor to further improve circuit efficiency.
It is, therefore, a primary object of the present invention to provide a circuit so that a low-voltage rated power switch can be used to improve circuit efficiency.
It is another object of the present invention to provide a circuit which recycles the leakage energy of a coupled leakage inductor to further improve circuit efficiency.
It is yet another object of the present invention to provide a circuit which uses as few components as necessary.
According to a first embodiment of the present invention, there is provided an active clamp step-down converter circuit with a power switch voltage clamping function including a first switch connected in series with an unregulated DC input source, a second switch coupled at one junction to a coupled winding having a first winding and a second winding, a leakage inductance associated with one winding of the coupled winding, a shottky diode connected in parallel with the second switch, and an active clamp circuit including a clamping capacitor and a third switch connected in series. The clamp circuit is connected in parallel with the leakage inductance and the first winding. The converter circuit further includes a filter capacitor connected to a coupled winding and in parallel with a load.
The clamping capacitor clamps the voltage across the first switch during the time in which the first switch is off. The clamped voltage across the first switch is the sum of the input voltage and clamping capacitor voltage.
According to a second embodiment of the present invention, there is provided a clamped step-down converter circuit with a power switch voltage clamping function including a first switch connected in series with an unregulated DC input source, a second switch coupled at one junction to one terminal of a coupled winding having a first winding, a second winding and a third winding, a leakage inductance associated with one winding of the coupled winding, a passive clamp circuit including a second diode whose anode is connected in series with one terminal of the third winding of the coupled winding and whose cathode is connected to the DC input source; a first diode whose cathode is connected to another terminal of the third winding and whose anode is connected to one terminal of the second winding of the coupled winding. A second terminal of the diode being serially connected to an anode of a second diode. The clamping circuit further includes a clamping capacitor connected in parallel with the first and second diodes and third winding of the coupled winding. The converter circuit further includes a filter capacitor connected at a midpoint of the coupled winding and connected in parallel with a load.
As in the first embodiment, the clamping capacitor clamps the voltage across the first switch during the time in which the first switch is off. The clamped voltage across the first switch is the sum of the input voltage and clamping capacitor voltage.
A main advantage provided by the present invention is the prevention or substantial elimination of voltage spikes which would otherwise occur at each switch transition to the OFF state. Voltage spikes are eliminated by incorporating the active clamp circuit in parallel with the first winding.
A further advantage of the present invention is that by recovering the leakage energy in each switching cycle, as opposed to dissipating it in accordance with prior art approaches, the overall circuit efficiency (i.e., power out/power in) is enhanced. An additional advantage of capturing the leakage current is that the voltage rating of the first switch is significantly reduced thereby reducing its cost.
A still further advantage;of the present invention is that the circuit is optimized to operate with a duty cycle of around 0.5 which improves the dynamic response and system efficiency. By operating with a nominal duty cycle of around 0.5, the circuit is responsive to changing load conditions. That is, when the load changes from a nominal to a heavy load, the duty cycle must be raised from 0.5 to a value close to 1.0 to insure that the output voltage variation remains within specification. Similarly, when the load changes from a nominal load to a light load, the duty cycle must be lowered from 0.5 to a value close to zero to insure that the output voltage variation remains within specification. The required changes in the duty cycle are most easily effected with a circuit that operates according to the present invention.